I. Field of the Invention
The present invention relates to a plasma separator and, more particularly, to a plasma separator through which blood is continuously flowed for filtering to separate the blood cells components, i.e., the packed blood cells solution, from the plasma.
II. Description of the Prior Art
As a method for separating plasma using a plasma separator of this type, a method for separating blood components by centrifugation is known. However, this method has various drawbacks. For example, the separation treatment rate per unit time is small, resulting in a long time and much labor for treatment of a large amount of blood. Furthermore, the separation treatment is complex in procedure, and the blood may be contaminated during the treatment.
Another plasma separating method is known which utilizes as a filter membrane a membrane with pores formed by bombarding a thin film of polycarbonate with charged particles.
Such a filter membrane has pores of precise size since the charged particles are used to bombard the thin polycarbonate film. However, the pores of the membrane may coincide in places. In order to prevent this, the porosity may only be made as high as 15 to 30%, so that a satisfactory filtering efficiency, thus, a satisfactory plasma filtration rate, may not be obtained. The polycarbonate used as the raw material of the membrane cannot remain flat when heat is applied thereto. The polycarbonate also allows many pinholes to form, limiting the range of application of this type of membrane. This type of membrane makes the structure of the plasma separator complex and bulky. Further, a membrane of this type cannot withstand use over a long period of time, and the drop in the plasma filtration rate with time is significant.
Still another plasma separating method utilizes a so-called hollow fiber membrane. A plasma separator using such hollow fiber membranes may be suitably used for a system in which blood is continuously flowed for separation, and the filtered blood is returned to the body from which the blood is taken.
More tension is exerted on the hollow fiber membranes during the manufacture thereof along the longitudinal direction, that is, along the direction of the channel length, than on flat membranes. Moreover, since there is no support means for supporting the membranes during manufacture, the shape of the pores formed becomes slightly oblong. Since it is difficult to maintain a uniform tension, it is difficult to obtain a uniform pore size. The pore distribution on the membranes thus becomes random, and precision in the manufacture of the membranes is considerably degraded. When performing the plasma separation using this type of membrane, the maximum pore size selected must be small in order to prevent leakage of the blood cells. As a result, the filtering efficiency of the blood cells components are significantly lowered. From the viewpoint of the structure, if the channel length of the blood channels is made short, the loss of the effective membrane area is enlarged and leakage occurs more frequently. If, however, the channel length is made long in consideration of this, the pressure drop increases, giving rise to hemolysis. Since the diameter of the hollow fiber cannot be made smaller than a certain limit imposed by the techniques involved in the manufacture of the membranes, the filtering efficiency per unit area of the membrane is lowered. When an attempt is made to make the membranes thinner in order to increase the plasma filtration rate, the strength of the membranes is lowered, leading to more frequent leakage of blood and variation in the pore size distribution of the membranes.
Under the circumstances as described above, the plasma separating method using the hollow fiber membranes has been unsatisfactory.
U.K. Pat. No. 1,555,389 discloses an ultrafilter in which a number of flat filtering elements are piled, each consisting of a substrate of open screen mesh and a membranous filtering medium covering the substrate. This patent shows the manner according to which the body fluid is filtered by this ultrafilter. Such a piled structure is preferable for the purpose of making the overall ultrafilter compact. However, this structure also has various problems. For example, it is difficult to make the flow path sufficiently thin, and the filtering membrane is low in wall shear rate, leading to an insufficient filtering efficiency. Also, the fluctuation in the filtration rate is great, leading to damages to the blood such as occurrences of hemolysis and coagulation. Further, the filter disclosed in the U.K. Patent, which is used for ultrafiltration, is incapable of filtering protein and, thus, is unsuitable for the plasma separation. In addition, leakage tends to occur in the case of separation of blood.
With a view to eliminating these disadvantages of the conventional device, there has been proposed a filter device which comprises a slender manifold plate provided at one end thereof with a blood inlet and at the other end thereof with a blood outlet, filter membranes attached to both sides of the manifold plate through the medium of interposed rubber gaskets incorporating 3 parallel channels, and collection plates provided with blood plasma vents and disposed so as to hold the filter membranes fast against the manifold plate [Trans. Am. Soc. Artif. Intern. Organs, XXIV, 21-26 (1978)]. Since the flow path has a great length compared with its width, this device entails a heavy pressure drop which gives rise to hemolysis and mars the practical worth of the device.
For plasma separation, the present inventor has made extensive researches on taking into consideration of microporous membranes manufactured by extrusion, stretching and so on. The membranes manufactured by these methods have more rough surfaces than the thin films of polycarbonate as described above. These membranes are also inferior to the polycarbonate membranes in terms of uniformity of pore size. For this reason, it has been considered that deterioration of the blood, such as hemolysis or coagulation, may be increased and plasma components may not sufficiently permeate the membranes. Accordingly, an improvement in the membranes for blood filtering is a matter of serious concern in this field.